The field of clinical chemistry and clinical analysis is concerned, inter alia, with the determination and quantification of various substances in body fluids. Many examples of substances which are to be determined can be given, and include cholesterol, urea, cations, and glucose. These examples of analyte as well as others, are assayed in diverse body fluids such as urine and blood.
One of the most frequently used devices in clinical chemistry is the test strip or dipstick. These devices are characterized by their simplicity of use. Essentially, the device is contacted to the body fluid to be tested. Various reagents incorporated into the device react with the analyte being determined to provide a detectable signal. Generally, this is a color or a change in color. These signals are measured or determined either visually or, more preferably, by an analysis machine. The detectable signal is correlated to a standard, so as to give a value for the amount of analyte present in the sample.
It will be understood that clinical analysis of the type described herein requires that any testing system be extremely accurate. In particular, when automated systems are used, it is essential to ensure that the elements of the analysis be reliable, and that the measurement taken be valid. It is for this purpose that control reagents are used.
Tietz et al., Textbook of Clinical Chemistry page 430, defines "control material" as "a specimen, or solution, which is analyzed solely for quality control purposes and is not used for calibration purposes". This standard reference work goes on to describe some of the requisite of a control material, as follows: "They need to be stable materials, available in aliquots or vials, that can be analyzed periodically over a long time. There should be little vial-to-vial variation so that differences between repeated measurements can be attributed to the analytical method alone". It must be added that the control material must be stable as well.
The cited reference, at page 433, discusses how the matrix of the control material should be the same as the material being analyzed. To that end, Tietz discusses modified human serum as one type of control material. Indeed, the art now recognizes the term "control serum" as referring to control material based upon serum. This terminology will be used herein, and is different from the term "control reagent" which, as used hereafter, refers to a control material which is not based on, and does not control serum of any type.
As has been pointed out, supra, one of the criteria which control materials have to satisfy is stability. Control materials based upon serum, however, are inherently unstable, due to the various components contained therein. Further, sera will vary from source to source, so uniformity from lot to lot cannot be guaranteed. Thus, it is sometimes desirable to have a control material based on a non-serum or serum free medium.
Examples of serum free control media, or "control reagents" as used herein, are seen in U.S. Pat. Nos. 4,684,615 and 4,729,959. The '615 patent teaches an aqueous isoenzyme control reagent. The reagent contains the isoenzyme of interest, together with other materials in a water base. More pertinent to the subject invention is the '959 patent, which is directed to "a stable glucose reference control". This control contains glucose in a range of from abut 40 to 500 mg/dl, together with fixed red blood cells, in an aqueous solution. The range of glucose concentrations given are sufficient to cover just about all ranges of glucose found in, e.g., blood.
The '959 patent points to a problem with aqueous control reagents at column 1, lines 50-55. Briefly, erythrocytes impart a degree of viscosity to blood which is absent in water based systems. This problem was also recognized in U.S. Pat. No. 3,920,580 to Mast. This patent teaches that aqueous solutions had not been consistent, and that a lack of reproduceability was observed when dry reagent strips were used with such materials. Mast taught that suitable reagents could be prepared using an antidiffusing agent in combination with glucose and water. The antidiffusing agents include polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, dextran, and bovine serum albumin. In U.S. Pat. No. 5,028,542, an invention is described wherein polystyrene sulfonates are added to aqueous glucose solutions. The polystyrene compounds are added so as to render the viscosity of the control reagents more similar to whole blood or serum.
An additional drawback to water based control reagents is exaggerated color formulation. The reaction which is monitored for analyte determination is generally one involving color formulation. Whole blood, of course, has a distinct red color, and serum is tinted differently than plan water. As a result, even when identical concentrations of the analyte of interest are present, and the same reaction system is used, a different degree of color formation results. While this problem can be addressed, in part, by use of suitable "control constants" or "control curves", it would be desirable to have control reagents available where the problem is eliminated or reduced.
The approach taken in the invention disclosed herein is somewhat similar to that of U.S. Pat. No. 5,028,542 in that polymers are used to address the problem of viscosity adjustment; however, the '542 patent does not disclose that polymers can be used to compensate for exaggerated color formation. Additionally, it is surprising that the polymers described herein possess the properties they do in control reagents, as they are not related to the polystyrenes of the '542 patent.
In a preferred embodiment of the invention, the polymers themselves are new. The polymers are based upon water soluble acrylic monomers, water soluble quaternary amines, or combinations thereof. The polymers are combined, in an aqueous solution, together with a predefined amount of a particular analyte of interest. The resulting material is useful as a serum free control reagent for use in determining unknown amounts of the particular analyte.